Optical Fiber Line, Optical Transmission Line, Method of Making Optical Cable, and Method of Laying Optical Transmission Line
1. Field of the Invention
The present invention relates to an optical fiber line for transmitting a plurality of wavelengths of optical signals in a wavelength division multiplexing (WDM) transmission system, an optical transmission line, a method of making an optical cable, and a method of laying an optical transmission line.
2. Related Background Art
Along with demands for higher speed and larger capacity in optical communications, wavelength division multiplexing transmission techniques for transmitting a plurality of wavelengths of optical signals as being multiplexed have come into use.
For enhancing the transmission quality of such wavelength division multiplexing transmission, optical fiber lines to become a transmission medium are required to have the following characteristics. As the absolute value of chromatic dispersion in the optical fiber line in a signal wavelength band (e.g., 1.55-xcexcm wavelength band) is greater, the pulse waveform of optical signals is more likely to deform, thereby deteriorating the transmission quality. Therefore, from such a viewpoint, it is desirable that the absolute value of chromatic dispersion in the optical fiber line be smaller. If the absolute value of chromatic dispersion in the signal wavelength band is smaller, on the other hand, then four-wave mixing, which is a kind of nonlinear optical phenomena, is more likely to occur, which causes cross talk and noise, thereby deteriorating the transmission quality. Therefore, from such a viewpoint, it is desirable that the absolute value of chromatic dispersion in the optical fiber line be greater.
For satisfying the two contradictory demands mentioned above, reference 1xe2x80x94K. Nakajima, et al., xe2x80x9cDesign of Dispersion Managed Fiber and its FWM suppression Performance,xe2x80x9d OFC""99 Technical Digest, ThG3 (1999)xe2x80x94, for example, discloses an optical fiber line whose chromatic dispersion is periodically changed so as to become positive and negative in the longitudinal direction, so that the absolute value of chromatic dispersion is sufficiently small in the line as a whole but locally greater. Described as a method of periodically changing the chromatic dispersion of optical fiber line so as to become negative and positive in reference 1 are methods in which core and cladding diameters are periodically changed, methods in which dopant concentrations are periodically changed, and the like.
The inventors have studied the conventional techniques mentioned above and, as a result, have found a problem as follows. Namely, the optical fiber lines in accordance with the above-mentioned conventional techniques necessitate a complicated manufacturing step in which the core or cladding diameter is periodically changed or a dopant concentration is periodically changed, which is also very hard to control. Along with the complexity in manufacturing steps and the difficulty in their control, the manufacturing cost rises as well. Therefore, in the case where an optical transmission line including a plurality of optical fiber lines is to be constructed in order to realize optical communications with a larger capacity, if the optical fiber lines in accordance with the above-mentioned conventional techniques are used therefor, then there will occur a problem that the manufacturing cost rises greatly.
Hence, it is an object of the present invention to overcome the above-mentioned problem, and provide an optical fiber line which has a higher transmission quality and can be constructed inexpensively, an optical transmission line, a method of making an optical cable, and a method of laying an optical transmission line.
The optical fiber line in accordance with the present invention comprises a plurality of positive dispersion optical fibers, having a positive chromatic dispersion in a signal wavelength band, selected from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A; and a plurality of negative dispersion optical fibers, having a negative chromatic dispersion in the signal wavelength band, selected from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; the positive and negative dispersion optical fibers being alternately arranged and coupled in a longitudinal direction.
Without any complicated manufacturing steps and difficult control, positive and negative dispersion optical fibers are alternately coupled in this optical fiber line, whereby the absolute value of cumulative chromatic dispersion can be made sufficiently small in the optical fiber line as a whole, whereas chromatic dispersion can be kept from locally becoming zero. Also, since the positive and negative dispersion optical fibers are selected from the positive and negative dispersion optical fiber groups, respectively, the cumulative dispersion in the optical fiber line as a whole can fall within a predetermined range if the average values DA and DB and the standard deviations "sgr"A and "sgr"B are appropriately adjusted.
Preferably, each of the positive dispersion optical fibers has a chromatic dispersion of 2 ps/nm/km or greater, whereas each of the negative dispersion optical fibers has a chromatic dispersion of xe2x88x922 ps/nm/km or smaller. In this manner, the absolute value of chromatic dispersion can be made locally greater.
Preferably, the average value of dispersion slope in the plurality of positive dispersion optical fibers and the average value of dispersion slope in the plurality of negative dispersion optical fibers have polarities different from each other. In this manner, a wavelength range in which the absolute value of chromatic dispersion becomes sufficiently small can be widened in the optical fiber line as a whole.
Preferably, each of the positive dispersion optical fibers and negative dispersion optical fibers has an effective area exceeding 50 xcexcm2. In this manner, nonlinear optical phenomena can be restrained from occurring.
Preferably, the absolute value of dispersion slope in each of the positive dispersion optical fibers and the absolute value of dispersion slope in each of the negative dispersion optical fibers are each smaller than 0.03 ps/nm2/km. In this manner, a wavelength range in which the absolute value of chromatic dispersion becomes sufficiently small can be widened in the optical fiber line as a whole.
Preferably, the ratio of the mode field diameter of any of the negative dispersion optical fibers to the mode field diameter of any of the positive dispersion optical fibers is at least 0.8 but not exceeding 1.2. In this manner, loss can be lowered at junctions between the positive and negative dispersion optical fibers.
Preferably, each of the positive dispersion optical fibers and negative dispersion optical fibers has a length of 5 km or shorter. Since the interval between two repeaters connected to each other by an optical fiber line is usually about several tens of kilometers, an optical fiber line in which a plurality of positive dispersion optical fibers and a plurality of negative dispersion optical fibers are alternately coupled will be laid between the two repeaters if each of the positive and negative dispersion optical fibers has a length of 5 km or shorter. As a consequence, even when chromatic dispersion fluctuates more or less among the individual optical fibers, the absolute value of chromatic dispersion in the optical fiber line as a whole can be made sufficiently small in terms of statistics.
An optical transmission line will be constructed if a plurality of optical fiber lines, each mentioned above, are contained therein.
In this optical transmission line, a plurality of optical cables are arranged adjacent each other in a longitudinal direction; each optical cable containing a plurality of positive dispersion optical fibers, having a positive chromatic dispersion in a signal wavelength band, selected from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A and a plurality of negative dispersion optical fibers, having a negative chromatic dispersion in the signal wavelength band, selected from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; wherein, in first and second optical cables adjacent each other selected from the optical cables, the positive dispersion optical fibers contained in the first optical cable and the negative dispersion optical fibers contained in the second optical cable are coupled to each other, whereas the negative dispersion optical fibers contained in the first optical cable and the positive dispersion optical fibers contained in the second optical cable are coupled to each other.
Alternatively, the optical transmission line may be configured such that a positive dispersion optical cable and a negative dispersion optical cable are alternately arranged adjacent each other in a longitudinal direction; the positive dispersion optical cable containing a plurality of positive dispersion optical fibers, having a positive chromatic dispersion in a signal wavelength band, selected from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A; the negative dispersion optical cable containing a plurality of negative dispersion optical fibers, having a negative chromatic dispersion in the signal wavelength band, selected from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; wherein the positive dispersion optical fibers contained in the positive dispersion optical cable and the negative dispersion optical fibers contained in the negative dispersion optical cable are coupled to each other.
Preferably, the optical cables provided in the optical transmission line are manufactured by the following method.
Namely, the method comprises the steps of selecting a plurality of positive dispersion optical fibers having a positive chromatic dispersion in a signal wavelength band from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A; selecting a plurality of negative dispersion optical fibers having a negative chromatic dispersion in the signal wavelength band from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; and using the plurality of positive dispersion optical fibers and the plurality of negative dispersion optical fibers so as to make a plurality of optical cables each containing positive and negative dispersion optical fibers.
Alternatively, the method comprises the steps of selecting a plurality of positive dispersion optical fibers having a positive chromatic dispersion in a signal wavelength band from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A; selecting a plurality of negative dispersion optical fibers having a negative chromatic dispersion in the signal wavelength band from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; and using the positive dispersion optical fiber so as to make a positive dispersion optical cable, and using the negative dispersion optical fiber so as to make a negative dispersion optical cable.
Preferably, the optical transmission line is laid by the following method.
Namely, the method comprises the steps of preparing a plurality of optical cables; each optical cable containing a plurality of positive dispersion optical fibers, having a positive chromatic dispersion in a signal wavelength band, selected from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A and a plurality of negative dispersion optical fibers, having a negative chromatic dispersion in the signal wavelength band, selected from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; and arranging the optical cables adjacent each other in a longitudinal direction such that, in first and second optical cables adjacent each other in the optical cables, the positive dispersion optical fibers contained in the first optical cable and the negative dispersion optical fibers contained in the second optical cable are coupled to each other, whereas the negative dispersion optical fibers contained in the first optical cable and the positive dispersion optical fibers contained in the second optical cable are coupled to each other.
Alternatively, the method comprises the steps of preparing a positive dispersion optical cable containing a plurality of positive dispersion optical fibers, having a positive chromatic dispersion in a signal wavelength band, selected from a positive dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DA ( greater than 0) and a standard deviation of "sgr"A; preparing a negative dispersion optical cable containing a plurality of negative dispersion optical fibers, having a negative chromatic dispersion in the signal wavelength band, selected from a negative dispersion optical fiber group whose cumulative dispersion conforms to a distribution with an average value of DB ( less than 0) and a standard deviation of "sgr"B; and alternately arranging the positive and negative dispersion optical cables in a longitudinal direction such that the positive dispersion optical fibers contained in the positive dispersion optical cable and the negative dispersion optical fibers contained in the negative dispersion optical cable are coupled to each other.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings. They are given by way of illustration only, and thus should not be considered limitative of the present invention.